The present invention relates to a color display system including a color cathode ray tube and more particularly to a color cathode ray tube having an electron gun structured so as to emit three electron beams toward a phosphor screen.
There are generally six kinds of potentials necessary in total to be supplied to a color cathode ray tube, such as a cathode potential, a control grid potential, an accelerating electrode potential, a focus electrode potential, an anode potential, and a heater potential for heating the cathode. FIG. 5 is a longitudinal sectional schematic view illustrating the rough constitution of a conventional electron gun. Numeral 01 indicates a heater having two terminals, 02 a cathode, 03 a control grid, 04 an accelerating electrode, 05 a focus electrode, and 06 an anode.
FIG. 6 is a perspective view illustrating the structure of a stem sealed to an end of an evacuated envelope of a color cathode ray tube for supplying each of the aforementioned potentials from the outside of the color cathode ray tube to each of the electrodes therein. Numerals 14 and 14' indicate stem leads and 17 a stem.
In FIGS. 5 and 6, voltages are applied to the heater 01, the cathode 02, the control grid 03, the accelerating electrode 04, and the focus electrode 05 from the outside via the stem leads 14, 14'.
A potential difference of 5 to 10 V is given across the two terminals of the heater 01 by two stem leads so as to flow a current of 200 to 700 mA through the heater.
A cathode potential which corresponds to a display signal is applied on the cathode 02 so as to generate a modulated electron beam and a voltage of about 0 to -200 V is applied on the control grid 03 as a control grid potential.
A voltage of about 200 to 1000 V is applied on the accelerating electrode 04 as an accelerating electrode potential and a voltage of about 5 to 10 kV is applied on the focus electrode 05 as a focus electrode potential. A voltage of about 20 to 35 kV is applied on the anode 06 as an anode potential.
The stem leads 14 are arranged at a regular interval between them, but the stem lead 14' for applying a high voltage of about 5 to 10 kV on the focus electrode 05 is located at a distance of two times the regular interval from the adjacent other stem leads 14 so as to prevent arcing between the stem leads.
The electron gun of the aforementioned constitution operates as described below.
Thermoelectrons emitted from the cathodes 02 heated by the heater 01 are accelerated by the accelerating electrode potential toward the control grid 03 so as to generate three electron beams. The three electron beams pass through the apertures of the control grid 03 and the accelerating electrode 04, are slightly focused by the prefocus lens formed between the accelerating electrode 04 and the focus electrode 05 before entering the main lens formed between the focus electrode 05 and the anode 06, and enter the main lens as accelerated by the focus electrode potential.
The three electron beams are focused respectively on the phosphor screen by this main lens to form beam spots.
A high voltage to the anode 26 is supplied Via the so-called anode button embedded in the funnel portion of the evacuated envelope of the cathode ray tube.
The prior art relating to such a color cathode ray tube is disclosed, for example, in Japanese Patent Application Laid-open Sho 59-215640.
There is a problem imposed in a color cathode ray tube having the aforementioned prior art electron gun that the resolution at corners of the screen (phosphor screen) is lower compared with that at the central portion of the screen.
The first reason of the main reasons is that a self-convergent deflection yoke is generally used so as to scan electron beams on the phosphor screen, and the astigmatism caused by the self-convergent deflection yoke increases due to the non-uniformity of the magnetic field and the second reason is that electron beam focusing conditions are different between the central portion of the screen and the corners of the screen because the distance from the main lens to the corners of the screen is longer than that from the main lens to the central portion of the screen.
To solve the problem that the resolution at the corners of the screen decreases, Japanese Patent Application Laid-Open Sho 61-250933 discloses a method in which a focus electrode comprises at least a first focus electrode and a second focus electrode, an electrostatic quadrupole lens is formed with the opposite surfaces of both the focus electrodes, and a dynamic voltage varying according to an increase in the deflection angle of an electron beam is applied on one of the first and second focus electrodes.
However, to apply the dynamic voltage, it is necessary to increase the number of stem leads by one. When the number of stem leads is increased in the stem of a limited size, a problem arises that the distance between the stem leads shortens and the so-called breakdown voltage characteristic that an arc is apt to occur between the stem leads due to a potential difference between each stem lead is degraded.